1. Field of the Invention
The present invention relates to a receiving apparatus applied to a digital satellite television broadcasting system etc., more particularly relates to an improvement of the channel search circuit.
2. Description of the Related Art
In recent digital satellite television broadcasting, different frequency bands are allocated to channels in for example the 20 MHz frequency bandwidth from 12.000 GHz to 12.020 GHz. However, the frequencies are not always fixed. This is because broadcasting services themselves are still not fully developed and there is a possibility of change of the bandwidth of the channels due to changes in the content of the services etc.
Further, since a satellite is utilized, there are cases where unusable channels are caused due to trouble in the transponder or other transmission apparatuses. For this reason, depending on the tuner for the digital satellite television broadcasting system, it sometimes becomes necessary to performing a full channel search at the user side at the point of time when there is a change in the channels at the time of purchase.
In this case, the carrier frequency of each channel is not always defined by the standards, so it is necessary to perform the demodulation operation while shifting the frequency little by little in the band.
A demodulation operation in digital television broadcasting generally takes a long time, however, unlike a demodulation operation in analog television broadcasting. This is because it is necessary to monitor a certain amount of the data to correct error. For example, in the case of the DVB (European Project Digital Video Broadcasting) system, a correct signal can be obtained only after going through processing such as QPSK (quadrature phase shift keying), Viterbi decoding, Reed Solomon decoding, etc. For this reason, a full channel search takes a considerably long time as a whole.